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RETIREMENT BENEFIT OBLIGATIONS - Sensitivity analysis (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
UK Plan      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Contribution to the plan $ 24.0 $ 32.0 $ 37.0
Supplementary payments 24.0 26.0 29.0
UK Plan | Discount rate      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Pension obligation +50bps/+1yr (80.9)    
Pension obligation -50bps/-1yr 92.6    
Pension cost +50bps/+1yr (2.0)    
Pension cost -50bps/-1yr 2.0    
UK Plan | Inflation      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Pension obligation +50bps/+1yr 88.1    
Pension obligation -50bps/-1yr (77.3)    
Pension cost +50bps/+1yr 2.0    
Pension cost -50bps/-1yr (2.0)    
UK Plan | Mortality      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Pension obligation +50bps/+1yr 38.0    
Pension obligation -50bps/-1yr (37.7)    
Pension cost +50bps/+1yr 1.0    
Pension cost -50bps/-1yr (1.0)    
US Plan      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Contribution to the plan 20.0 $ 20.0 $ 20.0
Supplementary payments 20.0    
US Plan | Discount rate      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Pension obligation +50bps/+1yr (25.5)    
Pension obligation -50bps/-1yr 27.9    
Pension cost +50bps/+1yr (1.0)    
Pension cost -50bps/-1yr 1.0    
US Plan | Mortality      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Pension obligation +50bps/+1yr 11.4    
Pension obligation -50bps/-1yr $ (11.6)